A preferred embodiment of the present invention is directed to a solar ultra violet (U.V.) radiation resistant, zinc coated polyvinylchloride (PVC) or related plastic pipe which may be integrated into a fluid transmission system which heretofore had been the exclusive domain of metal pipe, particularly stainless steel.
PVC, or related plastic materials, have been developed as a suitable material for fluid transmission, such as in enclosed environments, namely household or commercial plumbing systems, and for underground transmission systems, and the like. PVC, for example, is considerably less expensive than copper piping, it's light weight, readily fabricated and coupled, and it's corrosion resistant. However, a significant shortcoming of PVC, and related light-sensitive plastic pipe, insofar as exposed environmental applications are concerned, is its inability to resist solar U.V. breakdown. That is, PVC deteriorates under prolonged exposure to sunlight. It is theorized that there is a molecular breakdown caused by solar U.V. radiation. Such breakdown can manifest itself in a brittle or jagged failure of the PVC, which may be the result of loading. In contrast, an unaffected PVC, when overloaded, will exhibit a rather smooth break. There have been commercial attempts to prevent such breakdown by chemically or metallurgically modifying the PVC, such as by the inclusion therein of costly filler materials. However, such attempts defeat the purpose or goal of providing a low cost material resistant to solar U.V. breakdown.
Solar U.V. breakdown of plastic pipe is known from the teachings of Smith U.S. Pat. No. 2,918,394. This patent discloses a multi-layered product prepared by a complex fabricating procedure and consists of a pipe or hose made from two discrete layers, a first inner layer of substantially crystalline polyolefinic material, and a second outer layer comprising an organic polyisocyanate adduct. The outer layer has a high abrasion resistance and is adhesively bonded to the inner layer. According to the disclosure of this patent, many of the adducts are opaque and will not transmit sunlight, which sunlight causes ultraviolet breakdown of the inner layer. The patent indicates that excellent protection against sunlight can also be obtained by forming a surface deposit of metal, such as, for example, aluminum, on the inner layer by vacuum deposition. The metallic deposit acts as a mirror and can reflect all light, thus protecting the inner layer.
Some apparent limitations of the Smith product and fabrication procedure is that it requires vacuum deposition of metal, a procedure which naturally limits the product to small sizes, and is not applicable to any field work applications. Further, vacuum deposition of metal also does not form a good bond with the underlying plastic. It can be easily scratched or damaged and often will flake off leaving the plastic exposed. Presumably, that is why a protective coating must be applied over the plated surface, as stated in the patent.
Other prior art of interest is taught in Putter U.S. Pat. No. 4,120,325, and Langer U.S. Pat. No. 3,097,668. The patent to Putter discloses a practice for coating the surface of a pipe to promote electrical conductivity. Langer, on the other hand, refers to metallic spraying by the "flame" spray method which produces hot temperatures at the plastic surface due to high gas temperatures. This will melt the plastic. Langer goes on to state that metallic spraying would not be applicable to parts with curved surfaces. This is totally contrary to the present invention, and in fact teaches one skilled in the art away from the solution achieved herein.
The present invention, as more fully described hereinafter, relates to a cost effective conduit or pipe, and to the manufacture thereof, which pipe is rendered resistant to solar ultraviolet breakdown, and thus provide for environmentally exposed uses, such as a foam control spray system for use in a sewage treatment facility. That is, such system may comprise multiple PVC plastic pipes and spray nozzles coated with a diffusion bonded layer of metallic zinc or zinc alloy to prevent solar U. V. breakdown of such pipes and nozzles. Such invention will become more apparent from the following description, particularly when read in conjunction with the accompanying drawings.